1. Field of the Invention
2. Description of the Related Art
The present invention relates to tyres for motor vehicles and more specifically to their beads, i.e. those annular carcass zones which serve mainly to fix the tyre onto the corresponding mounting rim.
More precisely the abovementioned invention relates to the system for joining together the annular reinforcing structures contained inside said beads, more commonly referred to by the name of xe2x80x9cbead coresxe2x80x9d, and the ends of the carcass ply or plies.
A tyre traditionally comprises a toroidally shaped carcass having a central crown zone connected at its ends to a pair of axially opposite sidewalls which extend radially inwards and each terminate in a bead intended to fix the tyre to a corresponding mounting rim. For this purpose, the abovementioned reinforcing bead core is incorporated inside the bead.
The crown of said carcass has, coaxially arranged on it, a tread strip which is intended for the rolling contact of the tyre on the ground and is provided with a raised pattern defined by incisions and grooves which are formed in the thickness of said strip and are designed to ensure the handling properties of the tyre during use.
The reinforcing structure of the carcass comprises at least one ply of rubberized fabric consisting of a rubber sheet inside which textile or metal reinforcing cords are embedded, said cords being arranged transversely with respect to the circumferential direction of the tyre: in radial-carcass tyres the direction of the abovementioned cords is perpendicular to said circumferential direction, i.e. to the equatorial plane of the tyre.
Whatever the direction of the reinforcing cords, the ends of the carcass ply are folded back axially from the inside towards the outside around the abovementioned bead cores.
Owing to this geometrical arrangement, the profile of the carcass ply along the cross section of the tyre, and therefore also the profile of the corresponding reinforcing cord in radial-type carcasses, has an inflection point at the axially internal and radially external edge of the bead core, where the curvature of the abovementioned profile is reversed, changing from concave, along the sidewall, to convex, around the bead core.
In the continuation of the present description the abovementioned inflection point will be called the xe2x80x9cK pointxe2x80x9d.
When the carcass is of the radial type, it also comprises a belt structure arranged on the crown of the carcass, being located between carcass and tread strip and extending from one side to the other of the tyre, i.e. is substantially as wide as the tread strip.
The abovementioned structure traditionally comprises one or more pairs of strips of rubberized fabric provided with reinforcing cords which are parallel to one another in each strip and intersecting those of the adjacent strips, being inclined preferably in a symmetrical manner with respect to the equatorial plane of the tyre.
The tyre mounting rims have, at their axial ends, two more or less conical coaxial surfaces which form the support base for the tyre beads. The axially external edge of said bases terminates in a flange which extends radially outwards and acts as a support for the axially external surface of the bead and against which the abovementioned bead is kept pressed by the inflation pressure of the tyre.
The forced engagement of the tyre bead inside its seat is ensured by the conicity of the support base, which is open outwards, together with the metal reinforcing bead core, which is unextendable circumferentially and therefore has a constant diameter and is contained in the bead of the tyre: this forced engagement, which is produced by the axial thrust exerted on the side of the beads, axially from the inside towards the outside, by the inflation pressure of the tyre, ensures the stability of the tyre bead on the rim during use and, in the case of tubeless tyres, also airtightness between the tyre and rim so as to prevent gradual deflation of the tyre.
In order to guarantee fully this airtightness, the tubeless tyre is mounted on so-called drop center rims, i.e. rims made as one piece with bead support bases which have a high degree of conicity, i.e. are inclined at 15xc2x0 relative to the axis of rotation of the rim.
This particular inclination of the bead bases means that the tyre must have a particular profile of the carcass ply, distinguished by a tangent to said profile at the K point which is very inclined with respect to the axis of rotation of the rim.
All the above having been stated, the tyres to which the invention preferably relates are tubeless tyres for road transportation which have a metal, radial, single-ply carcass and which are used with working pressures of the order of eight bar.
In these tyres the bead zone is a structurally very critical area which very often results in disuse of the tyres owing to structural weakness in this zone well before the tread has become completely worn.
Many attempts have already been tried in the art in order to improve the situation, without, however, succeeding in solving the problem: these include, in particular, modification of the cross section of the bead core and that of the steel wire from which the bead core is made, if necessary also in combination with one another, in order to provide the bead core with greater torsional rigidity, as well as modification of the profile of the carcass ply and the attempt to optimize the value of the radial height of the folded-back portions of the carcass ply.
All these attempts have been unable to provide a completely satisfactory solution and have often given rise to problems which are even more serious than those which they were intended to solve.
In particular, the increase in torsional rigidity of the bead core has resulted in the simultaneous increase in its flexural rigidity, making mounting and removal of the tyre onto/from the rim extremely difficult: in turn, modification of the profile of the carcass ply has had a negative effect on many behavioural characteristics of the tyre during use.
According to Applicants"" perceiving the high inflation pressure not only induces in each cord of the carcass a pulling force which is transmitted onto the bead core along two components which are perpendicular to one another, i.e. an axial component and radial component, and which are opposed by the circumferential non-deformability of the bead core, but also applies to the bead core a twisting torque due to the distance of the K point from the center of mass of the bead core.
This twisting torque, during use of the tyre, varies with each rotational cycle of the tyre, producing cyclical microscopic movements in the whole structure of the bead and in particular microscopic rotations of the bead about its axially external edge and the edge of the flange, which result in the more or less rapid end of the life of the tyre.
It has been found that the problem may be advantageously overcome by causing the neutral profile of the carcass plies to pass inside the bead core, preferably through its center of mass, thus eliminating the inflection point.
The present invention relates to a new bead structure, in particular for giant-size tubeless tyres, which offers all the advantages of flexibility, elastic deformability and mechanical strength of the known beads, but which is devoid of the disadvantages illustrated hitherto.
The present patent also relates to a new bead structure which provides the tyre with a greater endurance.
Moreover the present patent relates to a new bead structure which provides the tyre with a smaller rolling resistance.
A different object of the present patent is a new bead structure which provides the tyre with a greater load capacity compared to known tyres which have the same dimensions.
A further different object of the present patent is a new bead structure which simplifies the tyre manufacturing process and allows the use of less costly machines.
Another different object of the present patent is a method for increasing the load capacity of a tyre for vehicle wheels compared to known tyres of the same size.
Another different object of the present patent is a method for eliminating the twisting torque, in the plane of the radial cross section, applied to the annular bead reinforcing cores in a tyre for vehicle wheels.
According to a first aspect thereof, the invention therefore relates to a tyre for vehicle wheels, comprising a torus-shaped carcass which has a central crown portion and two axially opposite sidewalls terminating in a pair of beads for fixing the tyre onto a corresponding mounting rim, each bead comprising at least one circumferentially unextendable annular reinforcing core, a tread strip which is located on the crown and coaxially extends around said carcass and is provided with a raised pattern for rolling contact with the road, and a belt structure coaxially arranged between said carcass and said tread strip, said carcass being provided with a reinforcing structure essentially consisting of at least one ply of rubberized fabric which is reinforced with metal cords lying in radial planes containing the axis of rotation of the tyre, said reinforcing structure having its ends secured to said annular reinforcing cores, and a neutral profile, in a radial cross-sectional plane, axially extending from bead to bead, characterized in that said neutral profile intersects the cross section of the zone which encloses said annular reinforcing cores and in that the ends of said reinforcing structure extend radially inwards not beyond the radially innermost profile of said annular reinforcing cores.
According to a second aspect thereof, the invention relates to a method for increasing the load capacity of a tyre for vehicle wheels, provided with a conventional carcass structure as described above, said method comprising the steps of limiting the radial extension inwards of the ends of the carcass structure so that they do not extend beyond the radially innermost profile of the annular bead-reinforcing cores, and of causing the neutral profile of the carcass plies to intersect the cross section of the zone which encloses said annular reinforcing cores.
According to a further aspect thereof, the invention relates to a method for eliminating the twisting torque, in the plane of the radial cross section, which is applied to the annular bead-reinforcing cores in a tyre for vehicle wheels as described above, said method comprising the steps of eliminating the inflection points along the extension of the neutral profile of the carcass reinforcing structure, and of causing said neutral profile to intersect the cross section of the zone which encloses said annular reinforcing cores.